As the dimension of a circuit pattern formed on a wafer becomes closer to and, further, smaller than the exposure wavelength used in lithography, effects of diffraction and interference of light appear remarkably. Consequently, a desired pattern is no longer transferred by simply forming a light-shielding pattern of the enlarged circuit pattern on a mask. Accordingly, techniques such as optical proximity correction (hereinafter, abbreviated as OPC) in which the shape of the pattern is modified or an assist pattern is added, for example, and a phase-shift mask that locally shifts the phase of the transmitted light by 180 degrees are widely used.
In addition to the techniques such as modifying the shape of the pattern, a technique called source mask optimization (hereinafter, abbreviated as SMO) is also carried out as an improving attempt for a source for lithography. In SMO, in order to form a required circuit pattern on a wafer, the optimum solution of the illumination condition of an exposure apparatus, the shape of a pixelated source, the mask design pattern, or the like is obtained through simulation using a high-speed high-capacity computer while taking into consideration characteristics of the exposure apparatus and an exposure process.
Patent Literature 1 and Non-patent Literature 1 disclose methods for optimizing a mask for lithography, using OPC and SMO. According to the methods, SMO is first performed on selected patterns in order to obtain an optimized source. Then, OPC with the source optimized through SMO is used to design the shape of a mask in accordance with a full-chip design.
However, in existing methods including the methods disclosed in Patent Literature 1 and Non-patent Literature 1, SMO and OPC are performed independently of each other, and consequently the source and the mask are optimized independently of each other. For this reason, a final mask pattern is not necessarily optimized by synergistically using advantages of SMO and OPC. Specifically, the shape of the source optimized through SMO is sometimes not optimum for OPC. That is, optimization according to the existing methods is sometimes not optimization of a source and a mask in which SMO and OPC are collaboratively used to exert a sufficient collaborative effect or are appropriately used in different manners. In particular, optimization is not performed in which whether to collaboratively use SMO and OPC is selectively decided for a so-called hotspot, which is a region including dense/complicated patterns, and for a non-hotspot.